A shotgun slug is a single projectile that is about as big as the bore of the shotgun. They can be rifled slugs, or sabot slugs- depending on the bore- rifled slugs for smoothbore, sabot slugs for rifled barrels. They are more accurate at longer ranges than the rifled slugs.

Buckshot refers of large round balls, between a quarter and a third of an inch in size. Depending on the guage and the buckshot size, there are different numbers of buckshot in one shell. Orginally used to deer hunt (buck). Typical 12 g may have 9-12 large buckshot. Number refers to size- smaller number, bigger shot. 000 is largest standard size (triple ought buck) but 00 (double ought) is most common.

Effectiveness depends on range. At 0-20 yards, 00 buckshot is hard to beat for effectiveness. At 100 yards, the slug.

As far as lethality- ANY firearm is capable of producing a lethal wound. There is no such thing as dead, deader, deadest.

in terms of killing power the 22lr is the most lethal round in the US. a better question is what will stop an aggressor the fastest??

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Well not quite correct. More people have been killed by a .22 than any other caliber but it is not the most lethal round.

The reason for the high death rate from a .22 is due to several things.
1. The sheer number of them. Every gun owner has a .22 of some type.
2. The time a .22 has been produced. The .357, .44 and others are relatively recent as far as history goes. The .22 goes way back over 150 years. In the early 1900's most men carried a .22 in their pocket and used them when a "need" arose.
3. The .22 will begin to tumble inside the human torso when it hits bone, muscle or such. The round may enter at the stomach and hit the heart, lungs or other. The size of the bullet is not the issue but rather the tumbling action tearing up organs.

As time goes on, there will be others over take the .22 and I look for it to be either the .40 or the .45acp. The reason for the .45acp doing so would be the use in military action. Right now, the 9mm is likely close to the .22 in the numbers of injury due to it's popularity with gangs, felons and such. People no longer take a .22 out to shoot others with. There are 9mm guns now so cheap that anyone can afford to own one and many do.
4.

As slug go I would not buy sabots for close range you won"t gain anything over slug and will need riffled barrle. If you are talking 75-100 yards the get a riffled barrel and go with sabots. If for home defense alternate buckshot and slugs, use a smooth bore.

What will make a bigger hole? depend on choke of the gun and what size shot. I would go with buckshot.